312 
COMPARATIVE ZOOLOGY. 
polished, overlapping scales, as in the Perch. Rarely, 
as in the Sturgeon, it is defended by bony plates, or by 
minute, hard spines, as in the Shark. Scales with smooth, 
circular outline are called cycloid; those with notched or 
spiny margins are ctenoid. Enameled scales are ganoid , 
and those with a sharp spine, like those of the Shark, are 
placoid. 
The vertical fins (dorsal, anal, and caudal) are peculiar 
to Fishes. The dorsal vary in number, from one, as in 
the Herring, to three, as in the Cod; and the first dorsal 
may be soft, as in the Trout, or spiny, as in the Perch. 
Fig. 2S4.—Blue-fish (Temnodon saltator). All seas. 
If the dorsals are cut off, the Fish reels to and fro. The 
caudal may be homocercal, as in ordinary species; or het- 
erocercal, as in Sharks. In ancient heterocercal Fishes, 
the tail was frequently vertebrated. The pectoral and 
ventral fins stand for the fore and hind limbs of other 
Vertebrates. As the specific gravity of the body is greater 
than that of the water, most Fishes are provided with 
an air - bladder, which is an outgrowth from the oesopha¬ 
gus. This is absent in such as grovel at the bottom, as 
the Rays, and in those, like the Sharks, endowed with 
compensating muscular power. 
Fishes have no prehensile organ besides the mouth. 
Both jaws are movable. The teeth are numerous, and 
